Christopher D. McQuaid

Gradients of intertidal primary productivity around the coast of South Africa and their relationships with consumer biomass

The structure of rocky intertidal communities may be influenced by large-scale patterns of productivity. In this study we examine the in situ rates of production by intertidal epilithic microalgae (chlorophyll a production per unit area per month), intertidal nutrient concentrations (nitrates, nitrites, phosphates and silicates), and standing stocks of different functional-form groups of macroalgae around the South African coast, and their relationships to consumer biomass. Clear gradients of in situ intertidal primary production and nutrient concentrations were recorded around the South African coast, values being highest on the west coast, intermediate on the south and lowest on the east coast. Primary production by intertidal epilithic microalgae was correlated with nutrient availability and could also be related to nearshore phytoplankton production. The dominance patterns of different functional forms of macroalgae changed around the coast, with foliose algae prevalent on the west coast and coralline algae on the east coast. However, overall macroalgal standing stocks did not reflect the productivity gradient, being equally high on the east and west coasts, and low in the south. Positive relationships existed between the average biomass of intertidal intertebrate consumers (grazers and filter-feeders) and intertidal productivity, although only the grazers were directly “connected” to in situ production by epilithic intertidal microalgae. The maximum body size of a widely distributed limpet, Patella granularis, was also positively correlated with level of in situ primary production. The maximal values of biomass attained by intertidal filter-feeders were not related to intertidal primary production, and were relatively constant around the coast. At a local scale, filter-feeder biomass is known to be strongly influenced by wave action. This implies that the local-scale water movements over-ride any effects that large-scale gradients of primary production may have on filter-feeders. The large-scale gradient in intertidal productivity around the coast is thus strongly linked with grazer biomass and individual body size, but any effect it has on filter-feeder biomass seems subsidiary to the local effects of wave action.

Phytoplankton production and biomass at frontal zones in the Atlantic sector of the Southern Ocean

A high resolution study of chlorophyll a and primary production distribution was carried out in the Atlantic sector of the Southern Ocean during the austral summer of 1990–91. Primary production (14C assimilation) and photosynthetic capacity levels at frontal systems were among the highest recorded during the cruise (2.8–6.3 mgC·m−3·h−1, and 1.3–4.7mgC·mgChl a−1·h−1, respectively). Blooms at ocean fronts were strongly dominated by specific size classes and species. This suggests that the increase in biomass was probably the result of an enhancement of in situ production by selected components of the phytoplankton assemblage, rather than accumulation of cells through hydrographic forces. This hypothesis is supported by the high variability of photosynthetic capacities at adjacent stations along the transects. Blooms (ca 2.7–3.5 mg Chl a·m−3) were found at three oceanic fronts (the Subtropical, Subantarctic and Antarctic Polar Fronts) during the early summer. These were equivalent to, or denser than, blooms in the Marginal Ice Zone and at the Continental Water Boundary. Seasonal effects on phytoplankton community structure were very marked. In early summer (December), netphyto-plankton (>20 μm) was consistently the major component of the frontal blooms, with the chain-forming diatoms Chaetoceros spp. and Nitzschia spp. dominating at the Subantarctic and Antarctic Polar Fronts, respectively. During late summer (February), nanophytoplankton (1–20 μm) usually dominated algal communities at the main frontal areas. Only at the Antarctic Polar Front did netphytoplankton dominate, with the diatom component consisting almost exclusively of Corethron criophilum. An early to late summer shift of maximum phytoplankton biomass from north to south of the Antarctic Polar Front was observed. Spatial covariance between silicate levels and water-column stability appeared to be the main factor controlling phytoplankton production at the Antarctic Polar Front. Low silicate concentrations may have limited diatom growth at the northern edge of the front, while a deep mixed layer depth reduced production at the southern edge of the front.

Marine alien species of South Africa — status and impacts

The current status of marine alien species along the South African coast is reviewed and the ecological and economic impacts of these invasions are discussed. In all, 10 confirmed extant alien and 22 cryptogenic species are recorded from the region. All 10 alien species support well-established populations and the majority of these remain restricted in distribution to sheltered bays, estuaries and harbours. Only one species, the Mediterranean mussel Mytilus galloprovincialis, has spread extensively along the coast and caused significant ecological impacts. These include the competitive displacement of indigenous species and a dramatic increase in intertidal mussel biomass. These changes have also increased available habitat for many infaunal species and resulted in enhanced food supply for intertidal predators. Considerable economic benefits have also resulted from this invasion because M. galloprovincialis forms the basis of the South African mussel culture industry.

Distribution of surface zooplankton and seabirds across the Southern Ocean

Abstract Surface zooplankton and seabird densities and community composition in the Atlantic (between Cape Town and Sanae) and Pacific (between New Zealand and the Ross Sea) sectors of the Southern Ocean are described and related to oceanographic features. Samples were collected during two return voyages aboard the MV Benjamin Bowring as part of the Transglobe Expedition (1979–1981). High abundances of surface zooplankton and seabirds were consistently observed within the main frontal systems of the Southern Ocean. Generally, on a mesoscale significant correlations between surface temperature and the distribution of zooplankton or seabirds were observed. On a macroscale, the geographical positions of the zooplankton and seabird communities coincided with specific water masses. The results of this study suggest that appropriate food availability rather than water temperature is important for the determination of seabird distribution. The ecological importance of the recently described frontal zone associated with the northern boundary of the maximum winter expansion of sea ice is confirmed by biological data obtained in this study.

Zooplankton structure and grazing in the Atlantic sector of the Southern Ocean in late austral summer 1993 Part 2. Biochemical zonation

The composition, biomass, distribution and grazing impact of dominant components of the meso- and macrozooplankton community were investigated along a transect occupied in the Atlantic sector of the Southern Ocean during the second cruise of the South African Antarctic Ecosystem Study conducted in late austral summer (Jan.–Feb.) 1993. Total zooplankton abundance along the transect ranged from 6 to 161 ind. m−3, and biomass levels ranged from 4. 2 to 80.5 mg DWt m−3. Elevated abundances and biomass were recorded at stations occupied in the vicinity of the Marginal Ice Zone (MIZ), the Antarctic Polar Front (APF) and Subantarctic Front (SAF). Throughout the investigation mesozooplankton, comprising mainly copepods, dominated numerically and by biomass accounting for >80% of the total. South of the APF, the copepods Calanus propinquus, Calanoides acutus and Metridia gerlachei numerically dominated zooplankton counts, while north of the front the zooplankton community was dominated by the copepods Calanus simillimus, Rhincalanus gigas and Lucicutia ovalis. Grazing impact of the six most abundant zooplankton taxa, accounting for up to 90% of all zooplankton counted at each station, demonstrated large spatial variability ranging from 0.01 to 18% of the chlorophyll (chl-a) standing stock or up to 89% of the daily phytoplankton production (DPP). The highest grazing impact along the transect was recorded within the Polar Front Zone where zooplankton removed between 4 and 18% of the chl-a standing stock or between 53 and 89% of DPP daily. Outside this region, zooplankton grazing generally corresponded to <5% of the integrated chl-a or <20% of the daily DPP. The implications of the spatial differences in grazing impact on the efficiency of the biological pump are discussed.

Non-climatic thermal adaptation: implications for species' responses to climate warming.

There is considerable interest in understanding how ectothermic animals may physiologically and behaviourally buffer the effects of climate warming. Much less consideration is being given to how organisms might adapt to non-climatic heat sources in ways that could confound predictions for responses of species and communities to climate warming. Although adaptation to non-climatic heat sources (solar and geothermal) seems likely in some marine species, climate warming predictions for marine ectotherms are largely based on adaptation to climatically relevant heat sources (air or surface sea water temperature). Here, we show that non-climatic solar heating underlies thermal resistance adaptation in a rocky–eulittoral-fringe snail. Comparisons of the maximum temperatures of the air, the snails body and the rock substratum with solar irradiance and physiological performance show that the highest body temperature is primarily controlled by solar heating and re-radiation, and that the snails upper lethal temperature exceeds the highest climatically relevant regional air temperature by approximately 22°C. Non-climatic thermal adaptation probably features widely among marine and terrestrial ectotherms and because it could enable species to tolerate climatic rises in air temperature, it deserves more consideration in general and for inclusion into climate warming models.

Thermal adaptation in the intertidal snail Echinolittorina malaccana contradicts current theory by revealing the crucial roles of resting metabolism

SUMMARY Contemporary theory for thermal adaptation of ectothermic metazoans focuses on the maximization of energy gain and performance (locomotion and foraging). Little consideration is given to the selection for mechanisms that minimize resting energy loss in organisms whose energy gain is severely constrained. We tested a hypothetical framework for thermal performance of locomotor activity (a proxy for energy gain) and resting metabolism (a proxy for energy loss) in energetically compromised snails in the littoral fringe zone, comparing this with existing theory. In contrast to theory, the thermal ranges and optima for locomotor performance and metabolic performance of Echinolittorina malaccana are mismatched, and energy gain is only possible at relatively cool temperatures. To overcome thermal and temporal constraints on energy gain while experiencing high body temperatures (23–50°C), these snails depress resting metabolism between 35 and 46°C (thermally insensitive zone). The resulting bimodal relationship for metabolism against temperature contrasts with the unimodal or exponential relationships of most ectotherms. Elevation of metabolism above the breakpoint temperature for thermal insensitivity (46°C) coincides with the induction of a heat shock response, and has implications for energy expenditure and natural selection. Time-dependent mortality is initiated at this breakpoint temperature, suggesting a threshold above which the rate of energy demand exceeds the capacity for cellular energy generation (rate of ATP turnover). Mortality in a thermal range that elevates rather than limits aerobic metabolism contrasts with the hypothesis that cellular oxygen deficiency underlies temperature-related mortality. The findings of this study point to the need to incorporate aspects of resting metabolism and energy conservation into theories of thermal adaptation.

Warming reduces metabolic rate in marine snails: adaptation to fluctuating high temperatures challenges the metabolic theory of ecology

The universal temperature-dependence model (UTD) of the metabolic theory of ecology (MTE) proposes that temperature controls mass-scaled, whole-animal resting metabolic rate according to the first principles of physics (Boltzmann kinetics). Controversy surrounds the models implication of a mechanistic basis for metabolism that excludes the effects of adaptive regulation, and it is unclear how this would apply to organisms that live in fringe environments and typically show considerable metabolic adaptation. We explored thermal scaling of metabolism in a rocky-shore eulittoral-fringe snail (Echinolittorina malaccana) that experiences constrained energy gain and fluctuating high temperatures (between 25°C and approximately 50°C) during prolonged emersion (weeks). In contrast to the prediction of the UTD model, metabolic rate was often negatively related to temperature over a benign range (30–40°C), the relationship depending on (i) the temperature range, (ii) the degree of metabolic depression (related to the quiescent period), and (iii) whether snails were isolated within their shells. Apparent activation energies (E) varied between 0.05 and −0.43 eV, deviating excessively from the UTDs predicted range of between 0.6 and 0.7 eV. The lowering of metabolism when heated should improve energy conservation in a high-temperature environment and challenges both the theorys generality and its mechanistic basis.

Spatial and temporal variability in recruitment of intertidal mussels around the coast of southern Africa

Intensity of intertidal mussel recruitment was compared across a range of different spatial and temporal scales around the coast of southern Africa between June 1995 and October 1996. Comparison of the east and west coasts revealed significantly higher recruit densities on the west coast, corresponding to larger adult densities. This difference between the two coasts reflects biogeographic disparities in mussel species composition, growth rates and spawning intensities, oceanographic conditions and productivity. Significant spatial variations in recruitment were recorded between regions 100–1000 km apart and between localities 1–25 km apart. Results suggest that the influence of dispersal processes on recruitment patterns acts at a relatively small scale, and may affect the distribution and abundance of adults among shores only a few kilometres apart. The high variability in recruitment intensities at a scale of metres indicates that larva! supply to the shore may be locally patchy, or that settlement preferences of recruits may be sensitive to subtle small-scale differences in adult density within mussel clumps. Small-scale differences in post-settlement mortality (e.g. owing to patchy predation pressure) may also play a role. This small-scale variability in recruitment is likely to reinforce the mosaic distribution of mussels evident on many of the shores. Significant temporal variability in recruit density was recorded, both between 3-monthly sampling intervals and interannually. Seasonal differences were absent for the north-west regions, whereas asynchronous seasonal patterns were displayed in the other regions. Results suggest that temporal cycles of recruitment are irregular and episodic, which may have important consequences for the dynamics of adults. Significant positive correlations were obtained between maximal recruitment and adult abundance, measured by density, or total number of adults on the shore (stock). This could be explained by the density-dependent role of adult conspecifics in providing suitable settlement habitat, or supply-side recruit limitation. These results have important implications for the management of exploited populations of mussels around the coast of southern Africa. Exploitation may influence recruitment success via at least two processes: reduction of larval supply by depletion of adult stock, and alteration of habitat suitable for settlement. Thus, overexploitation will compromise recruitment, which is itself the only mechanism of recovery. The west-coast mussel populations are likely to be more resilient to exploitation as recruitment is more predictable over time, stocks are larger and recruitment intensities high. This brings into question the present regulations for mussel harvesting because, paradoxically, more lenient regulations are applied on the east coast, where stocks and recruitment are low, than on the west coast, where biomass, recruitment and potential for recovery are high.

Enhancement of habitat heterogeneity and species richness on rocky shores inundated by sand

SummaryMany rocky shores are subject to periodic inundation by sand, which is often thought to reduce species richness by eliminating organisms intolerant of sand scour or sand smothering. However, regular disturbance (e.g. inundation) should promote richness by preventing the development of low diversity climax communities. A study of faunal richness on 10 regularly inundated shores showed that inundation does promote richness, but by increasing habitat heterogeneity. Some species are excluded from parts of the shore by sand, but because of the patchiness of sand deposits they are rarely excluded from the entire shore. Other species are found only on rocks associated with sand, while typically sandy shore animals occur in the sand deposits themselves. Total richness (281 species) was greater than for local noninundated shores and sandy beaches combined.